Latex-based paints have captured a significant portion of the indoor and outdoor paint market as a result of many advantages that such paints have over solvent-based products. The main advantages of latex-based paints include easy clean-up, low-odor and fast dry.
The term wet-adhesion is used in the paint industry to describe the ability of the paint to retain its adhesive bond under wet conditions. Good wet adhesion is well known in solvent-based paints, but water-based paints tend to lose adhesion in wet or humid environments. The use of water based emulsion polymer systems as protective and decorative coatings on previously painted oil based substrates, has become wide spread. However, there is a great need for improving the adhesion of water based coatings on oil based substrates.
Many efforts have been devoted in recent years to improving the wet adhesion of latex paints on previously painted alkyd substrates. This effort has involved attempts to optimize various paint formulation parameters, such as pigment types, dispersant types, surfactants and coalescing agents. Significant improvement in wet adhesion properties has been observed through modification of the polymer backbone of the latex binder with amine, amide or ureido functionality, U.S. Pat. Nos. 5,496,907; 4,617,364; and 4,319,032. In particular, cyclic ureido compounds have been described as imparting improved wet adhesion behavior, WO 97/49685. While other functionalities have also been known to improve wet adhesion, incorporating the ureido functionality on the polymer backbone has a much more significant impact on the wet adhesion properties of water based paints on alkyd substrates.
The precise mechanism for the wet adhesion of latex based coatings to alkyd based substrates has not been reported in the literature. In “Development of Ureido Functional Monomers For Promoting Wet Adhesion in Latex Paints”, R. W. Kreis, et al., the authors have suggested possible contributing factors for good wet adhesion of water based coatings on oil based substrates; i.e., 1) Anionic-Cationic interactions—Most of the substrates exhibit a net negative charge. By incorporating potentially cationic monomers, enhanced adhesive interactions can be provided between the functional group modified latex based coating and the negatively charged alkyd substrate. 2) Polar Interactions—Functional monomers can influence the degree of interaction between a latex film and the substrate. This could occur due to dipole interactions or hydrogen bonding. 3) Specific Bonding—A chelation type interaction between the latex binder and the substrate. 4) Interfacial Tension—Modification of the latex polymer with the appropriate functionalities can lower the interfacial tension between the polymer film and the substrate, thus enhancing the intimacy of contact.
The effects of various functional groups on wet adhesion were examined by making polyurethane-acrylic hybrid polymers containing different functional monomers, and then formulating them into a semi-gloss paint formula. Results obtained with functional monomers copolymerized with an acrylic latex were compared to systems with the functional monomer incorporated in the polyurethane backbone, and to systems without any functional monomer. The polyurethane-acrylic hybrids, with and without wet adhesion monomers, were prepared as disclosed in U.S. Pat. No. 6,031,041, incorporated herein by reference. A schematic representation of the resulting structure of these polyurethane-acrylic hybrids is shown in FIG. 2.
The wet adhesion properties of a polyurethane-acrylic hybrid without functional monomer were compared to a typical blended dispersion. Similar wet adhesion performance was obtained for the hybrid and the blend. The hybrid failed after 370 cycles, while the blend failed after 350 cycles
Hybrid dispersions were prepared with the ureido group attached to the acrylic latex. Significantly better wet adhesion was observed. The film did not fail until 2000 cycles, compared to 370 cycles obtained for the hybrid without any functional monomers. A second ureido hybrid was prepared, but in this case, the ureido group was incorporated into the polyurethane backbone. This system also exhibited excellent wet adhesion, and did not fail until 1500 cycles.
Finally, hybrid dispersions were prepared with dimethylaminoethyl methacrylate and silane functional monomer attached to the acrylic latex, instead of the ureido functional monomer. The wet adhesion results for the dimethlyamine functionality and for the silane monomer were compared to the results obtained for the ureido functionality. These results are shown in FIG. 3. This plot shows that the silane functionality did not significantly improve the wet adhesion (380 cycles to failure for the silane system compared to 370 for the hybrid without functional monomer). A slight improvement in wet adhesion was observed for the hybrid containing the dimethylamine functional monomer (450 cycles to failure); however, the improvement was not nearly as large as that obtained with the ureido functional monomer (2000 cycles to failure). The surfactant stabilized acrylic latex resulted in 280 cycles.
Molecular modeling provides detailed information about molecular structure, electronic structure and molecular interactions. Therefore, molecular modeling can be used to determine the importance of these effects and may be able to explain some of the observed wet adhesion behavior.
In order to compare the polarity of the various functional groups in the polyurethane dispersion, high quality electronic structure calculations were performed on models of the various chemical moieties found in the polyurethane. The structures of the model compounds are shown in FIG. 4. The dipole moments of a urea, urethane and ureido group were computed and compared.
The molecular geometry and electronic structure of the model compounds were obtained using ab initio local density functional quantum mechanics. This was calculated using the Becke-Perdew-Wang (BPW) functional within the DMOL computer program, available from Accelrys Inc. A double numeric polarization basis set was also used in all calculations. Partial atomic charges, bond orders and dipole moments were all obtained using the DMOL quantum mechanics computer program.
The computed values of the dipole moments of the urethane, urea and ureido model compounds are 2.48, 3.86 and 3.56 Debyes, respectively. These results indicate that the ureido group is slightly more polar than the urethane group, but is less polar than a urea group. Therefore, molecular polarity alone does not explain the improved wet adhesion imparted by the ureido functionality. A more detailed modeling approach that simulates polymer-alkyd molecular interactions is required.
Surprisingly it has been found that a molecular model based on molecular interaction modeling can accurately predict the adhesive results of the tested systems. This modeling method may be used to predict the adhesive interactions between any polymer and a surface. The method of the present invention was presented at the International Waterborne, High-Solids, and Powder Coatings Symposium in the paper “Molecular Modeling of Adhesion Promoting Monomers for Coatings”, Farwaha, et al, 2001.